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Pressure tank sizing for lake pump

I am looking to add a pressure tank to my irrigation setup which is fed from a dedicated 1.5hp centrifugal lake pump.
Basically I just want the tank to keep my mainline pressured so that my valves will stay closed when the pump is off and to sense a "demand" event and start the pump. Currently my setup requires you to turn the pump on manually and then open the valve. Not terrible, but not ideal. Also, when my pump is off for awhile the pressure behind my valves drops low enough the the diaphram release and the valve opens. When I turn the pump back on the valves are flowing for 5-10 secs before the pressure closes them again. Annoying to say the least.
I am not looking for the tank to actually supply water during use, I want my pump to start on the demand event, and continually run until all outlets are closed and pressure returns to 42 psi at which point it should stop the pump.
I was considering a FloTec horizontial 6-gal pre-charged unit (note: I don't want to mount it on the pump, but outside the garage). I called FloTec tech support to confirm I can use this tank outdoors and then asked about sizing and the gal read to me from the "script" that I need to size it based on the GPM of the pump which is 60+ GPM, which would me 1 VERY big tank or multiple tanks. I tried to explain how my use is slightly different than normal, but she couldn't deviate from her script.
Am I right that for my scenario I can get away with a small tank that would provide enough pressure to push against my valves and the pump (check valve) and keep my line pressurized (albeit low pressure) and then when a valve (or spigot) opens it would turn the pump on which would continue to run until the valve was closed and the pressure returned to a set amount (ie. 42 psi or thereabouts)? Do you think the 6-gal tank I mentioned would work?
I assume that my pressure switch selection is likely more important than the tank. Any advice on this? I don't know what my operating pressure range would be, I've only measured "closed". I haven't finished my zones zet so won't know all my "open" pressure values right away. I assume there are switches that allow adjusting the range (start and stop psi)?
Thanks!

The script has a point in that absolute pump protection from too-rapid cycling is a combination of flow rate and tank size, which, along with pressure switch settings, determines the minimum possible interval between pump starts. All that said, for sprinkler zone operation, all the pump flow goes to the sprinkler heads, and there is no cycling, and a small tank will serve the purpose of keeping upstream pressure in the mainline.

Where the script comes back into play is when you use a hose connected to the pressurized mainline. Since you will never get a hose to use the full pump flow, when the pump is operating in the pressure range set by a pressure switch, you are going to get cycling, and too small a tank will make it very rapid cycling. Any current hose operation from that pump is at the very edge of its operating curve, and you can't safely use pressure switch settings at that edge, because you will be very close to the switch not shutting off the pump at all, and then you are deadheading. A larger tank will increase the period of cycling with hose operation, and that provides pump protection.

Based on a quick scan of tank prices, you might look at a pre-charged tank in the 32-40 gallon range as a starting point, in order to have hose bib operations without worries over pump life. Local tank prices might favor a 20 gallon tank, which I consider a bare minimum. You can visit pump/tank tutorials and work out the numbers yourself. When you called tech support, they work from an understandable assumption that your pump flow is a constant number, which is what you generally get with deep-well submersible pumps. On a centrifugal pump, the flow rate decreases as the pressure rises, so you can pick an arbitrary operating point at least 10 psi away from its deadheading pressure, and use a flow that is far less than 60 gpm.

I was going to check the pressure in the line when the spigot is running. I actually have dual spigots so this is fairly easy to do. Based on that I was hoping that my "open" (either spigot or zone valve) range would still allow me to set the pressure switch to cover either possibility without the switch shutting off the pump.
I may be ignorant of reality (highly likely). I assumed that if my pressure with everything closed and the pump running was 42psi, I could set my shutoff at about 40psi. I thought that with "anything" open I would never achieve 40psi and the switch would remain on, even if the tank refilled. The conjecture on my part is that once I "close" everything the tank is full and the 40psi reached, the pump would shutoff and the pressure tank would keep the mainline at 40psi. Now that I say it out loud, I realize I am not talking about a start/stop range, but a start/stop threshold. Above it off, below it on. Is this possible? Is this flawed?
Came across this device which sounds like exactly what I need, but of course is comes at a cost http://www.cyclestopvalves.com/simple/irrigation.php?anim=1above
Any experience with something like this out there?
Thanks for the help!

Every device you put in the system will subtract pressure, and you do not have pressure to spare with a centrifugal pump. Since you chose a pump already and are sticking with it, you accommodate the system to it, and invest in a tank.

I certainly don't want to add any additional devices to my system, ecspecially at that kind of price tag.

Doing the math will never replace real world experience which is why I turned to this forum, but I appreciate your point. As for building character, thanks for looking out for me, but I am doing fine

The online calculators are basically the same as the "script". They are designed to size a tank to deliver water to the system and reduce pump cycles (less run time is better). The calculators recommend very large tanks for my scenario, which I know is unnecessary overkill. I think the size you recommended is more practical, but the calculators would never have even come close to that unless I made up the numbers to get the answer I wanted.

Assuming the lowest psi when running a zone is 20, and the highest psi when runnnig just a spigot is 30, I "assume" I will be able to set a switch (Square-D for example) to cut-in at 30 and cut-out at 40, by adjusting the differential setting to tighten the range and then adjust the range to match the target. If I went with a 35 gallon Flotec tank it is charged at 40 psi, which is the top end of my range. I think I would need to bleed off about 20 psi to get its empty psi to about 10-20. It will likely require some tweaking to get the tank to be at about 30-35 gals and 40+ psi, but once I achieve this it should hold at the cut-off range (30-40psi) just fine for a couple of days before losing enough through attrition to require a "bump" back to the top by running the pump for a minute or two.

Now this is all just theory as I have never worked with any of this equipment before and I would certainly appreciate a "heads-up" if I am going to be dissapointed with the results after I spend hundreds of dollars and hours of time to install it.

This post has been edited 1 times, last edit by "dharwood" (Sep 24th 2014, 7:43am)

The "I Told You So Department" would like to take a moment to remind everyone that it was recommended that the centrifugal pump get traded in on a jet pump, which could become a stable water supply with the addition of a Cycle Stop Valve, a pressure switch, and a baby pressure tank, and all for less cost than was spent on the centrifugal and the flow sensor. The performance curve on that pump makes it a very touchy thing indeed, to have it operating at a pressure that would allow as little as a 10 gpm flow, since that pressure would be very close to where it would be deadheading. Maybe you will have to supplement the larger tank with some interface with the flow sensor you mentioned (make and model number, please)

I don't know where the businesses that take pumps on trade-in are, but I am not aware of any in my neighborhood. The research I did pointed to the centrifugal pump as being the correct choice for irrigation/sprinkler applications. Sprinklerwarehouse.com has this exact quote when referring to centrifugal pumps... "A vast majority of all irrigation pumps fall into this category". I wish I would have posted the question on this forum before buying and using the pump, but I didn't. Can we move on from this?

I have used the pump for a three years now and am very happy with it. It has been supplying a single 3/4" garden hose for this whole time at a very low flow rate and hasn't deadheaded. I have been told that these pumps are used in this way frequently and do just fine, but are certainly capable of more.

I didn't want to use a pressure tank at all and would just rely on a pump start relay, however I learned the hard way that I need the lines pressurized to keep the valves closed. Without some pressure in the line, the zones will run for 5-10 secs every time I turn the pump on which is not desirable.

So I find myself in a situation where I have already made a commitment to this setup and am trying to make the most of it with some help from those experienced in this field. I'll understand if you do not want to help me, because you feel I have painted myself into a corner with bad choices, but beating me over the head about it isn't helping.

So not worrying about deadheading...should I be able to set up a pressure tank and switch in such a way (describe in previous post) that I will be able to have the pump come on when a demand event occurs (or shortly after) and run continuously until all valves are closed again (or shortly after)? I realize that the factory configuration and standard settings are not going to work for me, but I'm wondering if I can lower the pressure in the tank and narrow the range in the switch to make it work?

You have to remember that these threads are often read by others trying to learn about equipment choices, and it's for the now and future benefit of a wider audience that I stress some of these points, because every day, someone looks at a cast-iron "lawn sprinkler pump" and thinks that's just the item needed for a sprinkler system installation, not knowing the designation is either outdated or downright deceptive. Today's common rotor sprinklers need much more pressure than the simple brass spray heads of yesteryear, and a system greatly benefits from having a water source with more pressure than a centrifugal pump can manage.

So trust me, no one's beating up on you. It just happens that you walked into a teachable moment. Smile!

Meanwhile, your pump is going to be performing as its curve describes, and to make the combination of pressure switch and pressure tank work for you, you will be fine-tuning the cut-off pressure adjustment, so that you get a full minute of pump operation every time it starts, on the way to the tank pressure going back up to the cutoff point. This is where we get back to the script that was being read to you, and what the larger tanks make possible.

The larger the tank, the lower your cutoff pressure can be, so long as you are having the same cycle interval. You want an interval of at least one minute as a safe minimum, and a pro setting up a deep-well submersible can be thinking in terms of two or three minutes between pump starts. So this is where you have a tough choice to make, because this is an application where bigger is better. The only downside to a bigger tank is that it costs more money. The downside to a tank that is too small, is that it won't be able to give you a full minute of runtime for your pump.

The pressure switch itself is a fairly simple mechanical item (I recommend a Square D Pumptrol FSG2) and they have their limitations, in that if you set a cutoff point of 43 psi, it will most of the time cut off at 43 psi and at other times may cut off one or two psi higher or lower. This detail is important because the absolute maximum cutoff pressure you might set for your pump is 47 psi, the pressure that it would be deadheading at. (see the chart)

So the bigger the tank you buy, the more wiggle room you give to your pressure switch adjusting. And you need that wiggle room, because the difference in output pressure with you running a hose, and the pump simply deadheading is going to be a very small difference. What works in your favor, is that a cycling pump with you running a hose will take a bit longer to reach cutoff pressure (but not so much longer that it makes up for an undersized tank)

Here's a simpler calculator to play around with, but no online calculator will take into effect that your pump flow rate is going to start out big and end up small. The selection of a tank is going to come down to your budget and willingness to invest in a reliable level of pump protection. See what tanks are available in your area, and compare prices and capacities and reviews of quality. I think you should start the search with a 32-36 gpm precharged tank.

Thanks Wet_Boots. Very clear explanation of the situation and options.

I think I have what I need and understand the challenge I face depending on the choices I make.

The part I feel isn't being taken into account is that I WANT my pump to run. I don't "need" water delivered from the tank. My purpose for the tank is to keep the line pressurized when the pump is off. Without it my line pressure drops to zero and my valves won't function correctly. The calculators are geared towards trying to deliver a significant portion of the demand without having to run the pump.

For instance, the calculator you linked to has the first question "Pump output in GPM?". Mine is roughly 60+. If I enter that, is says I need to park a tanker truck in my back yard to match that volume without running the pump. This particular calculator tops off at 30gpm. If I enter 30gpm, 1.5 second run time (continuous isn't an option) and 30/40 cut in/out, it says I need a 250 gallon tank. So I then answer the pump output question with 5 gpm and I get a more reasonble sized tank (42). But that is just me knowing how to trick the calculator to give the answer I think I need. I understand my use is not what the calculator is designed for. Heck, I understand that my use is not what the tanks and switches are designed for. Typical recommendation for my setup is to use a Pump Start Relay.

If I had never tried to expand my sprinkler pump into a irrigation system pump, I would still have been using it to pump to a single sprinkler, so as far as "pump protection" goes, my worst case scenario is my starting point. With what I am trying to do I will eventually do the majority of my watering using the zones and valves which will be higher flow, lower pressure, happier pump. Given that, my goal is to simply introduce a tank/switch to pressurize the line when the pump isn't running, and wait patiently when it is.

So with the information you have provided, which I greatly appreciate, I understand that it IS possible but NOT guarenteed because I will likely be trying to balance very narrow pressure margins. Additionally, in my low flow situations my pump with be close to deadheading pressure (again I am already in this situation). I'm ok with that.

I feel compelled to say, for all those that may come across this thread because they searched for "sprinkler pump" or "centrifugal pump". This 1.5 hp, 110v, 2" intake, 1.5" output pump has been the best purchase we have made in the past 20 years for trying to water our vacation property. After countless headaches with burnt out pumps that lost prime and never really delivered enough performance, this pump has been rock solid, has never lost prime and delivers MORE than enough water to every corner of our property. If you are considering this pump for just running sprinklers via a hose (or multiple hoses) then I highly recommend it. If you are considering it for delivery to an irrigation system, then ask the pros before making a choice.

You are both complicating and oversimplifying the situation. The over-simplification comes from your statement that you are pumping 60 gpm. What you are actually pumping is a flow rate that can be found on the performance curve. That curve starts at zero gpm and goes all the way to 88 gpm. In terms of your pump operation and pressure switch settings, your pump will begin operating at near to 80 gpm when it cuts in, and by the time it cuts out, you can hopefully have it fine-tuned to be pumping at 10 gpm or less. If you really wanted to mathematically work out precise cycling times from the performance curve, you would have to use calculus.

The complicating comes from assuming the tank is in place to deliver water. Not the case. It's just there to both provide some constant upstream pressure for the sprinkler zone valves, and to keep the pump from cycling too rapidly during operations other than lawn sprinkling. If you only cared about the constant upstream pressure, you could have a pressure switch and baby tank mounted right on the pump itself. The sprinkler system would be happy with only that.

The problem for you is that you also want to have the ability to draw random intermittent small amounts of water from hose bibs, fed by a pump that would cycle a pressure switch very rapidly, unless sufficiently sized pressure tanks are a part of the system. To have an install of your particular centrifugal pump that was proof against any and all possible operating situations might involve so much pressure tank capacity that it would actually be cheaper to replace the pump with a small jet pump, and work out the tank and pressure switch details from there.

It may work out that your best, and simplest, solution will be to employ a Cycle Stop Valve, and have it, the pressure switch, and the baby pressure tank on the pump itself or nearby. That will subtract some additional pressure, but you'll live with the loss, rather than have a tank farm.

{we interrupt this post for another teaching moment}

A jet pump is what this application really calls for, and we only need to see some jet pump performance curves to know why. Instead of unused overcapacity, you have a useful range of pressure and flow that graphs as nearly a straight line, for your calculating pleasure. The 1/2 HP jet pump would do fine in your application with a 20 gallon size precharged pressure tank, and there wouldn't be any agonizing over tweaking the pressure switch adjustment just right. And no Cycle Stop Valve needed.

{end of teaching moment}

And as for your own make and model of centrifugal pump, it possesses a key characteristic of all reliable electric pumps, which is a motor with a high service factor. More than anything else, that "service factor" is why some pumps endure and other burn out, assuming everything else is equal. The cast-iron jet pumps selected by conscientious professionals will also possess high service factors.